The present invention relates to an electrochromic device having improved light stability.
Electrochromic devices are already known, for example from D. Theis in Ullmann""s Encyclopaedia of Industrial Chemistry, Vol. A 8, page 622 (Verlag Chemie, 1987), and WO-A 94/23333. A distinction is made between two basic types:
Type 1: full-area electrochromic devices;
Type 2: electrochromic display devices having structured electrodes.
Type 1 is used, for example, in electrically darkenable window panes or electricaly dimmable automobile mirrors. Such devices are disclosed, for example, in U.S. Pat. No. 4,902,108.
Type 2 is used in segment and matrix displays. Such display devices are proposed, for example, in DE-A 196 31 728. Devices of this type can be observed transmissively or, in the case of reflection, reflectively.
WO-A 94/23333 compares electrochromic materials having different constructions, but these are not used as display devices:
Construction a: the electrochromic substances are in the form of a fixed film or layer on the electrodes (see Ullmann, above).
Construction b: the electrochromic substances are deposited on the electrodes as a layer by the redox process (see Ullmann, above).
Construction c: the electrochromic substances remain permanently in solution.
For construction (a), the best-known electrochromic material is the tungsten oxide/palladium hydride pair.
For construction (b), viologens have been described as electrochromic substances. These devices are not self-erasing, i.e., the image produced remains after the current has been switched off and can be erased again only by reversing the voltage. Such devices are not particularly stable and do not allow a large number of switching cycles.
In addition, the cells constructed using tungsten oxide/palladium hydride in particular cannot be operated in transmitted light, but only reflectively, owing to light scattering at these electrochromic layers.
Elektrokhimiya, 13, 32-37 (1977), 13, 404-408, 14, 319-322 (1978), U.S. Pat. No. 4,902,108, and U.S. Pat. No. 5,140,455 disclose an electrochromic system of construction (c). An electrochromic cell built up from glass plates with a conductive coating contains a solution of a pair of electrochromic substances in an inert solvent.
The pair of electrochromic substances used is one electrochemically reversibly reducible substance and one reversibly oxidizable substance. Both substances are colorless or only weakly colored in the ground state. Under the action of an electric voltage, one substance is reduced and the other oxidized, both becoming colored. When the voltage is switched off, the ground state re-forms in the case of both substances, decolorization or a color lightening taking place. 
U.S. Pat. No. 4,902,108 discloses that suitable pairs of redox substances are those in which the reducible substance has at least two chemically reversible reduction waves in the cyclic voltammogram and the oxidizable substance correspondingly has at least two chemically reversible oxidation waves.
According to WO-A 94/23333, however, such solution systems of construction (c) have serious disadvantages. Diffusion of the electrochromic substances in the solution causes fuzzy color boundaries and high power consumption in order to maintain the colored state, since the colored substances are permanently degraded by recombination and reaction at the opposite electrode in each case. Nevertheless, various applications have been described for such electrochromic cells of construction (c). For example, they can be formed as automobile rear-view mirrors which can be darkened during night driving by application of a voltage and thus prevent dazzling by the headlamps of following vehicles. See U.S. Pat. No. 3,280,701, U.S. Pat. No. 4,902,108, and EP-A 0435 689. Furthermore, such cells can also be employed in window panes or automobile sunroofs, where they darken the sunlight after application of a voltage. Likewise described is the use of such devices as electrochromic display devices, for example in segment or matrix displays having structured electrodes. See DE-A 196 31 728.
The electrochromic cells normally consist of a pair of glass plates, of which, in the case of the automobile mirror, one is mirrored. One side of these sheets is full-area coated with a light-transparent, electroconductive layer, for example, indium-tin oxide (ITO), where, in the case of display devices, this conductive coating is divided into electrically separated segments provided with individual contacts. These sheets are used to construct a cell by bonding them by means of a sealing ring with their electroconductively coated sides facing one another to form a cell. This cell is filled with an electrochromic liquid through an opening and the cell is tightly sealed. The two sheets are connected to a voltage source via the ITO layers.
The electrochromic devices described above generally exhibit sensitivity to light, in particular UV light. Electrochromic devices containing UV absorbers have therefore been described, for example, in U.S. Pat. No. 5,280,380.
Compared with the use of UV absorbers, the use of electrochromic compounds which inherently have better light stability would be advantageous.
Surprisingly, it has now been found that the use of certain dihydronaphthazines or dihydrophenazines results in improved light stability of the electrochromic device.
The invention accordingly relates to an electrochromic device comprising
(a) a pair of glass or plastic plates or plastic films wherein at least one such plate or film (preferably both plates or films) is provided on one side each with an electrically conductive coating, wherein
(1) at least one such plate or film and its conductive coating is transparent,
(2) the other such plate or film and its conductive coating is optionally mirrored,
(3) the electrically conductive layer of one or both of the two plates or films is optionally divided into separate segments optionally provided with individual contacts, and
(4) the plates or films are joined on the sides of their conductive coating by means of a sealing ring to form a volume; and
(b) the volume formed by the two plates or films and the sealing ring is filled with an electrochromic medium comprising a pair of electrochromic substances OX2 and RED1, wherein
(1) OX2 is a reducible electrochromic substance, and
(2) RED1 is an oxidizable electrochromic substance represented by at least one of the formulas 
xe2x80x83wherein
R201 is aryl,
R202 is alkyl, cycloalkyl, alkenyl, aralkyl or aryl,
B is a bivalent bridge,
m and n, independently of one another, are integers from 1 to 4, and
R203 to R206, independently of one another, are hydrogen, halogen, alkyl, alkoxy, cyano or aryl,
with the provisos that when m is at least 2, two adjacent R203 together also optionally represent a bivalent xe2x80x94CHxe2x95x90CHxe2x80x94CHxe2x95x90CHxe2x80x94 radical and when n is at least 2, two adjacent R204 together also optionally represent a bivalent xe2x80x94CHxe2x95x90CHxe2x80x94CHxe2x95x90CHxe2x80x94 radical.